In recent years, particulates such as soot contained in exhaust gases discharged from internal combustion engines of vehicles such as buses, trucks and the like and construction machines have raised serious problems as those particulates are harmful to the environment and the human body.
There have been proposed various ceramic filters which allow exhaust gases to pass through porous ceramics and collect particulates in the exhaust gases, thereby purifying the exhaust gases.
Conventionally, with respect to the honeycomb filter of this type, a filter having the following structure has been proposed in which: two kinds of through holes, that is, a through hole with a relatively larger capacity (hereinafter, referred to as large-capacity through hole) and a through hole with a relatively smaller capacity (hereinafter, referred to as small-capacity through hole) are prepared, and the end on the exhaust gas outlet side of the large-capacity through hole is sealed with a plug, with the end on the exhaust gas inlet side of the small-capacity through hole being sealed with a plug, so that the surface area of the through hole with the opened inlet side (hereinafter, referred to as inlet-side through hole) is made relatively greater than the surface area of the through hole with the opened outlet side (hereinafter, referred to as outlet-side through hole); thus, it becomes possible to suppress an increase in pressure loss upon collecting particulates (for example, see Patent literature 1, and FIG. 17 of Patent literature 2).
Moreover, another filter has been disclosed in which: the number of the inlet-side through holes is made greater than the number of the outlet-side through holes, so that the surface area of the inlet-side through holes is made relatively greater than the surface area of the outlet-side through holes; thus, it becomes possible to suppress an increase in a pressure loss upon collecting particulates (for example, see FIG. 3 of Patent literature 2).
In the case of the honeycomb filter used in filters for purifying exhaust gases, disclosed in Patent literature 1 and Patent literature 2, in comparison with a honeycomb filter in which the total amount of the surface area of the inlet-side through holes and the total amount of the surface area of the outlet-side through holes are the same, since the surface area of the inlet-side through holes is relatively greater, with the result that the deposition layer of collected particulates becomes thinner, thereby making it possible to suppress an increase in a pressure loss at the time of collecting particulates.
Moreover, after having collected a predetermined amount of particulates, an engine controlling process is carried out through a post injection system or the like to raise the exhaust gas temperature and the temperature of a heater placed on the upstream side of exhaust gases from the honeycomb structural body is raised so that, upon burning particulates, the particulates are made in contact with high-temperature gases to be easily burned, making it possible to accelerate the burning speed of the particulates.
However, in the above-mentioned conventional honeycomb filters, ashes that remain as dregs after particulates have been burnt are accumulated on the wall face of the through holes as they are without being moved. For this reason, the problems with the above-mentioned structures are that pores, formed in the partition wall, are closed and that the ashes tend to form bridges to cause clogging in the through holes, resulting in an abrupt rise in the pressure loss.
Moreover, in the case of the honeycomb filter shown in FIG. 17 of Patent literature 2, as the surface area of the large-capacity through holes is made relatively greater, the weight of the honeycomb structural body constituting the honeycomb filter tends to decrease, resulting in a reduction in the thermal capacity and the subsequent good thermal response. Consequently, the burning speed of particulates becomes too fast, with the result that ashes are deposited on the wall faces of the through holes, as they are, without being moved, and the ashes tend to form bridges to cause clogging in the through holes, resulting in an abrupt rise in the pressure loss.    Patent literature 1: Patent gazette No: 3130587    Patent literature 2: U.S. Pat. No. 4,417,908 (FIG. 3, FIG. 17 and the like)